Move helper functions in PopupMenuImpl.cpp.
Move helper functions in PopupMenuImpl.cpp.

* Move CSS value serializer functions to the top of PopupMenuImpl.cpp, and wrap
  them with anonymous namespace.
* Also, do not use |default:| in |switch|es.

This CL has no behavior changes. This is a prepration of a large change of
PopupMenuImpl.cpp.

BUG=513339

Review URL: https://codereview.chromium.org/1256793007

git-svn-id: bf5cd6ccde378db821296732a091cfbcf5285fbd@199563 bbb929c8-8fbe-4397-9dbb-9b2b20218538

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